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48 www.resourceworld.com a u g u s t / s e p t e m b e r 2 0 1 5 MININGWORLD M ining activities don't always involve blasting and excavat- ing large surface or underground cavities. In cases where the commodity of interest is soluble and can be dissolved in water-based solutions it is possible for its extraction directly from the ground with minimal disturbance to the surface environment. Copper, uranium, lithium, potash, rock salt and boron are, under certain circumstances, amenable to non-invasive mining methods called in-situ (in place) leaching (ISL), in-situ recovery (ISR), or solu- tion mining. Injection wells are drilled and a dilute leaching solution is pumped into the orebody. The solution moves through the ore- body in a controlled manner and dissolves minerals of interest. The resulting pregnant solution is then recovered through a series of surrounding recovery wells for processing through chemical pre- cipitation processes or solvent extraction-electrowinning. After recovery, the fluid is recirculated through the ISR circuit in a new cycle of leaching. Monitoring wells are located beyond the capture zone. They are designed to ensure that all solution is contained and recovered. Aquifer water is neutralized, filtered and treated by reverse osmosis at closure time. These mining methods are nothing new. Deep drilling techniques were developed 2,500 years ago by Chinese trying to access and exploit underground brine deposits. The flammable natural gas found associated with these deposits was collected and transported through bam- boo pipes and used for heating the recovered brines to accelerate evaporation. In 907 AD the Chinese were also documented as being the first to use ISL to produce cop- per. The first uranium ISL operations were developed in the early 1960s in the US and Russia. there are numerous advantages to isR when com- pared to conventional mining methods: The method allows for economic mining of low-grade ore deposits and of deposits that are not economic to be mined through underground methods that are not acces- sible through open pits; • Lower capital and operating costs; • Smaller size deposits could be mined at profit; • Not a water intensive operation as water is recycled and reused; water consumption is a fraction of the amount used by agriculture operations; • Easier to permit and build – an estimated 1.5 years to permit and one year to build vs 3-7 years for a conven- tional mine to be permitted and built; • No mining per se takes place – no large cavities are created – no need for waste dumps, leach pads or tailings facilities; • Closure costs are less than for hardrock mining. the most important limitations to isR methods are: • The type of mineralization – oxide (and carbonate) copper and uranium mineralization only are suitable for in-situ leaching (not sulphides); • The host rocks need to be highly fractured and the fissures interconnected in the case of copper ISR; or porous in the case of uranium ISL; • Most of the copper mineralization has to be found as coatings on fissures, in veinlets and not disseminated throughout the host rock; • The orebody has to be located below the water table and, in the case of uranium ISL, it should be hosted by aquifers confined by impermeable strata to protect water quality; • Copper mineralization should not be hosted in carbonate rich In-situ recovery of metals and minerals by Dan Oancea Illustration courtesy of Excelsior Mining Corp.